The potability of backcountry water is uncertain. Though it is sometimes believed that natural sources of water in backcountry or wilderness areas are clean and potable, and sometimes they are, actually this water may be unsafe to drink.

Large rivers may be tainted with pesticide runoff and industrial pollutants from sources far upstream, but water in the backcountry, where people usually go for hiking or backpacking, originates nearby and is free from these hazards. The most common danger is microbial, and this may come from natural or human sources.

In most parts of the world, water may contain bacterial or protist contamination originating from human and animal waste, or sometimes from dead animals in or near the water. Giardia lamblia and Cryptosporidium spp., both of which cause diarrhea (see giardiasis and cryptosporidiosis), are common pathogens. Viruses may also be found in water, but are not common in developed countries.[citation needed]

Is giardiasis a threat in outdoor recreation?

Robert Rockwell, an engineer by training, quotes James Wilkerson's Medicine for Mountaineering and Other Wilderness Activities. (The Mountaineers, 4th edition, 1992):

"In recent years, frantic alarms about the perils of giardiasis have aroused exaggerated concern about this infestation. Government agencies, particularly the U.S. Park Service and the National Forest Service, have filtered hundreds of gallons of water from wilderness streams, found one or two organisms (far less than enough to be infective), and erected garish signs proclaiming the water hazardous."

Rockwell also quotes two researchers who surveyed health departments in all states and scanned the medical literature looking for evidence that giardiasis is a significant threat to outdoor people:

"Neither health department surveillance nor the medical literature supports the widely held perception that giardiasis is a significant risk to backpackers in the United States. In some respects, this situation resembles (the threat to beachgoers of) a shark attack: an extraordinarily rare event to which the public and press have seemingly devoted inappropriate attention." (Welch, Thomas R. and Welch, Timothy P.: Giardiasis as a Threat to Backpackers in the United States: A Survey of State Health Departments. Wilderness and Environmental Medicine, Vol. 6, 1995)

In his exhaustive article, Rockwell concludes that untreated surface water in the Sierra Nevada is generally safe to drink. He notes that "Giardia and other intestinal bugs are for the most part spread by direct fecal-oral or food-borne transmission, not by contaminated drinking water. Since personal hygiene often takes a backseat when camping, the possibility of contracting giardiasis from someone in your own party someone who is asymptomatic, probably is real. Recalling that up to 7 percent of Americans, or up to 1 in 14, are infected, it is not surprising that wilderness visitors can indeed come home with a case of giardiasis, contracted not from the water...but from one of their friends."[1]